If you want something simpler, look at rc4. There are alot of rc4.c code examples out there, just google for "rc4 c". There is a Crypt::RC4 module, and you might want to look at it's perldoc.

I think your biggest problem will be making sure the keys are the same. Many encryption algorithms pad the keys to be a certain length, and you will need to make sure you do it the same in c and perl, or it won't work.